In a computer system, electrical devices such as a hard disc, a motherboard and a power supply are disposed inside the housing to avoid the radiation of electromagnetic waves affecting the normal operation of adjacent elements. To avoid an unused hard disc slot being exposed and causing the electrical devices in the housing under the interference of electromagnetic waves, a cartridge is normally inserted into the unused hard disc slot. Suppose four hard disc slots are not in use, the user would need to purchase four cartridges to interpose the four unused slots. This solution not only incurs additionally cost but also requires complicated operations of loading and unloading the cartridges, which is indeed very inconvenient.